The invention relates to systems and processes for reactivating poisoned, noble metal-containing catalysts, and more particularly to the reactivation of platinum-containing catalysts used in measuring the concentration of organic gases in air.
Scientists and engineers have developed a number of devices that exploit the effects of catalytic reactions. For example, today there are catalytic monitors designed to measure the concentration of organic gases in air, and to detect the existence of hazardous conditions. These monitors can reduce the risk of accident by detecting the presence of explosive gases which can build up or be released at public utilities, propane distributors, fire services, HVAC contractors, landfill operators, steel mills, natural gas buses and other similar locations. These instruments typically include an element that is generally referred to as a pellistor or pelement. A typical pellistor consists of a small ceramic bead cast on a coil of wire, where the wire serves as both a heater and a thermometer. Electronic circuitry determines the resistance of the element and hence the pellistor""s temperature rise or the decrease in power required to maintain the pellistor at a constant temperature when exposed to a gas containing a combustible constituent In effect, the heat of oxidation of the analyte is measured and, through a calibration procedure, related to the quantity of the analyte present in the gas stream. A detailed discussion of pelements and catalytic combustible gas sensors which include such pelements is found in Mosely, P. T. and Tofield, B. C., Solid State Gas Sensors, Adams Hilger Press, Bristol, England (1987).
Pellistor catalysts usually contain palladium, platinum, or a mixture or alloy that includes at least one of these two noble metals. Palladium catalyzes methane combustion in air at a lower temperature than platinum. Platinum, when maintained at a temperature sufficient to combust methane, is less susceptible than palladium to poisoning by sulfur. These characteristics have resulted in palladium sensors being preferred for battery operated equipment where power consumption must be minimized and platinum sensors being preferred for fixed gas detention systems where long life is desirable.
The chemical process catalyzed by the pellistor is the oxidation of an organic gas in air to yield mostly water vapor and carbon dioxide. Although such catalyst systems can work well initially, deposits can build up and remain on the catalyst surface and, in time, decrease the pellistor""s sensitivity. Such deposits may form if the gas contains molecules or atoms that are not readily converted to vapors by oxidation. Such molecules or atoms are referred to as catalyst poisons. Commonly encountered poisons for palladium include sulfur-containing organic compounds such as odorants, organohalides, organosilicons, organoleads, and organophosphates. Typically, only organosilicons, organoleads, and organophosphates act to poison pellistors catalyzed by platinum. To address the problem of catalyst poisons, a filter may be incorporated around or on the pellistor. Such pellistors can absorb a finite amount of poison and may have a some-what extended life, but will be poisoned by repeated or high level exposures.
Once poisoned, the catalyst is generally ineffective in detecting the presence of combustible gases. This renders the catalytic device unusable. Consequently, catalytic poisoning is a costly problem as it can destroy the usefulness of expensive catalyst systems such as gas detectors, as well as systems for converting conversion or reforming of petroleum feedstocks into other chemical compounds and catalytic air cleaning systems for automotive exhaust gases, which can lose activity due to exposure to the oxidation resistant poisons often contained in engine lubricants. For example, W. H. Preston et al. in the Institution of Mechanical Engineers Papers, Conference on Vehicle Emissions and Their Impact on European Air Quality, 1987-88, has shown a statistical link between the phosphorous content of lubricants and the catalyst performance of automobile air cleaning systems.
Further troubling is that poisoning from metalloids such as boron, silicon, germanium, arsenic, and antimony can resist existing recovery techniques. Specfically, metalloid-poisons form polymeric oxides which are not converted to gases by heating in oxygenated environments. Consequently, although both non-metal and metalloid-containing organic compounds poison noble metal catalysts, the polymeric metalloid oxides cannot be removed by oxidation. Accordingly, there is need for a recovery process that can treat catalysts poisoned by metalloid compounds.
Accordingly, it is an object of the invention to provide processes that restore catalytic activity for catalysts poisoned by metalloid containing compounds.
It is another object of the invention to provide systems that allow for in situ treatment of pellistors housed within portable or fixed gas detection systems.
Other objects of the invention will be discussed or made apparent from the following description of the invention.
The invention provides methods to reactivate a poisoned noble-metal catalyst, such as a pellistor that includes a noble metal. The catalyst may have been poisoned during field use due to exposure to gases of unknown and uncontrolled composition, or in the laboratory by exposure to air mixed with a compound that includes a metalloid. The methods can consist of contacting the poisoned catalyst with a non-oxidizing, hydrocarbon containing environment under conditions sufficient to achieve a reactivation reaction that restores, in part or in whole, the catalytic activity of the poisoned catalyst. The conditions to achieve this effect can include temperatures that are sufficiently high to achieve the reactivation reaction and, optionally, at temperatures that are sufficiently low enough to avoid, or reduce, sintering effects in the catalyst. In one practice, the catalyst is heated to between about 400xc2x0 C. and 750xc2x0 C. The heated catalyst is exposed to a gas stream containing a hydrocarbon such as methane, ethane, or ethylene, either substantially pure or mixed with an inert gas such as nitrogen or argon. In one practice, the hydrocarbon mixture is in concentration of about 14% to 100% hydrocarbon to 86% to 0% inert gas. The gas stream provides, or is part of, a non-oxidizing environment which lacks any substantial amount of oxidizing agent, such as air or oxygen.
More specifically, the processes described herein recovers catalytic activity of a noble metal catalyst by exposing the catalyst to a hydrocarbon-containing, non-oxidizing environment, and controlling the temperature at which the catalyst is exposed to the hydrocarbon-containing, non-oxidizing environment to reactivate substantially portions of the catalyst that have lost activity. The reactivated catalyst can be reemployed within gas detection system, or within another catalytic device.
The term xe2x80x9cnon-oxidizing environmentxe2x80x9d as employed herein will be understood to encompass any environment that will not support substantial combustion, and will include any environment that has an oxygen content of approximately zero to two percent, and more preferably less than one percent.
The term xe2x80x9chydrocarbon containing environmentxe2x80x9d as used herein will be understood to encompass any environment that comprises, in part or in whole, a hydrocarbon compound, including saturated or unsaturated compounds, including any alkanes, alkenes, alkymes, cyclic or ring compounds, branched-chains or derivatives, and for example shall be specifically understood to include methane, ethane, propane, butane and ethylene.
The possible temperature and pressure characteristics of the processes are manifold, and any suitable conditions for achieving the desired reactivation reaction can be employed and are to be understood as within the scope of the invention. Further, the mechanics for achieving the selected environment can vary and can include, for example, heating directly the catalyst, or placing the catalyst within a heated enclosure, or alternatively heating the gas stream.
The systems described herein include systems that provide for the in situ recovery of catalytic activity of pellistors and other catalytic elements employed within gas detector systems, air cleaning systems and other devices. In one embodiment the systems include catalytic gas detectors that have a combustion sensor including a pellistor having a noble metal catalyst. The detectors include a controller capable of operating in a detection mode for monitoring a characteristic of the pellistor, which varies in response to the concentration of a particular gas within a flow of gas. The controller can also operate in a regeneration mode for heating the pellistor to an operating temperature selected to regenerate catalytic activity of the pellistor upon exposure of said pellistor to non-oxidizing, hydrocarbon containing environment. The catalytic gas detector can include a controller that has a regeneration mode for heating the pellistor to between about 400 and 700xc2x0 C. and preferably approximately 640xc2x0 C. The controller can include a microprocessor operating responsive to a set of instruction signals including instruction signals for directing the controller to heat the pellistor to a temperature sufficient to reactivate catalytic activity. Optionally, the controller can include a timer for allowing a time period to be selected during which time period the pellistor is maintained at a temperature, or cycled through a set of temperatures, sufficient to reactivate catalytic activity.
In other embodiments, the system can be a stand alone unit that includes a chamber which is dimensioned to receive a pellistor, or a device having a pellistor, and to create an environment suitable for achieving a reactivation reaction that recovers catalytic activity of the pellistor.
Although the systems and methods described herein can be employed for recovering activity of a catalyst having lost activity due to any poisoning material including phosphorous, and silicon, it will also be understood that a further aspect of the invention includes methods and systems for recovering the sensitivity of a catalyst that has been poisoned by a metalloid-containing compound, or any compound upon oxidation that forms a polymeric oxide. The methods can include exposing the catalyst to a non-oxidizing, hydrocarbon containing environment, and controlling the temperature at which the catalyst is exposed to the non-oxdizing, hydrocarbon containing environment to achieve a temperature sufficient to reactivate portions of the catalyst poisoned by the metalloid containing compound.
The term xe2x80x9cMetalloid containing compoundxe2x80x9d as used herein will be understood to include any compound comprising an element having properties intermediate between a metal and a non-metal, boron, silicon, germanium, arsenic, tellurium, polonium antimony and any comparable element Consequently, methods described herein include methods for recovering catalysts that have lost activity from compounds that are deposited as residue on noble metal catalysts, including residue deposited during combustion with oxygen rich gases, and will include catalysts that have lost activity from oxides of metalloid containing compounds that are polymeric and are not readily converted to gases by heating.
The catalysts capable of being treated by the systems and methods described herein include any catalyst comprising, in part or in whole, noble metals including platinum, palladium, silver, iridium, rhodium, ruthenium, and osmiumn and mixtures thereof. These noble metals are typically associated and supported on or with a metal oxide and particularly an oxide of a metal in the left-hand columns of Groups III to VIII of the Periodic Table, including oxides of silicon, aluminum, titanium, zirconium, hafnium, thorium, vanadium, tantalum, chromium, molybdenum, tungsten, uranium, manganese, zinc, colbalt, and nickel. It will be understood that the catalyst can comprise two or more noble metals and/or two or more metal oxides, and that activating or other compounds can be included within the catalyst as well.
The objects and advantages of the systems and methods described herein include the restoration of pellistor catalyst activity without sintering the catalyst or otherwise reducing its active surface area. In particular, the sensitivity of pellistors poisoned by metalloid-containing compounds may be recovered by the methods and systems described herein. Furthermore, some pellistors that lose sensitivity after prolonged periods of use in the field may be restored by a short treatment period including treatment periods of five to ten minutes. Moreover, maintaining appropriate control of the reaction conditions may allow the pellistor to be reactivated without being removed from the analytical instrument in which the pellistor is installed. Consequently, application of the systems and methods disclosed herein may quickly and conveniently restore the lost sensitivity of a pellistor.
Other aspects and embodiments of the invention will be apparent from the following description of certain illustrative embodiments.